I use Finale Print Music to make my sheet music and it is a great affordable software to get you started. Finale notepad is a light version which contains basic functionality if you want to try Finale for a small cost of $20.

Pianoteq is the first 4th generation piano. It isn't a sampler that plays back a recording of a note, but it is a simulator of a piano. When you hit a key on a MIDI controller or a MIDI keyboard, you are hitting a virtual key which moves a virtual hammer that strikes a virtual string. This allows the player to play with feeling as if they were using a real piano. I use this software on my MIDI keyboard during performance, recording, or arranging.